This invention relates to the protection of laser receiving devices from damage by an incident beam of radiation of excessive intensity and, more particularly, to a protection system employing a plurality of electric arcs disposed across a path of incident radiation and electrically energized in series from a single power source.
Radiation receiving devices are employed in a variety of situations. Of particular interest are receiving devices employed in the imaging of scenes emitting radiation such as infrared radiation An infrared imaging system employs, typically, an array of radiation sensitive detectors which view incoming radiation via an optical focusing system including a scanning mirror The detectors output signals in response to the incident radiation, the detector signals being employed by well-known signal processing circuitry to produce an image of the scene upon a display for viewing by persons operating the infrared imaging system.
Radiation detectors, of the type employed in an array of many detectors may be fabricated of semiconductor material which is susceptible to damage, as by heating, when exposed to excessively strong radiation. For example, a laser beam which might be directed inadvertently or deliberately toward the receiving optics of an imaging system could inflict significant damage to the radiation detectors so as to disable the detectors.
Such detectors, or sensors of radiation, are found in laser rangefinders, radars, and passive receiving systems such as the aforementioned imaging system. The receiving systems employ receiving telescopes which may have a relatively large field of view and a large acceptance angle through which laser radiation may be received. In the case of an active device, such as a laser rangefinder, it is intended that the receiving telescope receive laser radiation. However, in the case of a passive infrared imaging system, it is intended that only scene radiation be incident upon the receiving telescope even though the telescope is responsive to laser radiation at the infrared frequencies.
The laser radiation can be at a variety of wavelengths, and may be pulsed or continuous wave. In order to protect a receiving device from excessively strong laser radiation, a fast-operating optical switch is desired to close the optical path in the presence of the strong radiation, which switch must allow the normal intensity radiation to pass without interference through the optical system to the detectors. Such an optical switch must be activated almost instantaneously prior to the damaging of the detectors, to block or greatly reduce the transmission of strong laser radiation. Ideally, such an optical switch should protect against a broad band of laser wavelengths, and be operative over a wide field of view.
One type of switch which has been shown to offer protection from excessively strong radiation is the plasma arc switch. An understanding of the operation of a plasma arc in blocking a path of laser radiation is explained in U.S. Pat. No. 3,964,003 issued in the name of Cohn et al on June 15, 1976, and U.S. Pat. No. 4,380,079 issued in the name of Cohn et al on Apr. 12, 1983.
High density plasma can interact with radiation by refraction, reflection and absorption. The use of this effect to divert laser radiation for purposes of pulse shaping and power limiting has been demonstrated in the first of the foregoing patents. For example, radiation from a 5 micron wavelength carbon monoxide laser can be diverted by a short pulse plasma arc placed at a beam focal spot. The use of intense-localized, high current arcs is required in order to reach the plasma densities necessary to divert or absorb laser radiation.
A problem arises in the use of a plasma arc switch due to the use of relatively large fields of view in radiation receiving equipment. The large fields of view would require an array of many electric arcs through plasma, rather than a single arc. Heretofore, the generation of numerous arcs would require numerous sources of power and difficulty in connecting the power sources to the sites of the various arcs without interference with the optical transmission properties of a receiving telescope. In addition, the use of multiple power sources may present a problem of synchronization of the arcs such that all arcs are generated in time to protect the detectors. By way of example, it is envisioned that a number of arcs, on the order of 10-20 arcs would be required in a typical system. Such a number of arcs would require, heretofore, an excessively large number of power cables, enlarged total amount of power, and would entail electromagnetic interference generated by the various currents applied to the arcs. As a consequence of such a complex arrangement, system efficiency, system size, and compatibility with sensor electronics would be significantly compromised resulting in a system which could not be employed effectively in situations requiring minimum size and high performance as in an airborne or satellite imaging system.